Automated molding technology for thermoplastic injection molding

ABSTRACT

A method for the automated optimization of an injection molding machine set-up process comprising injection molding one or more parts, inspecting the parts for defects, adjusting the injection stroke and/or the injection velocity and repeating the process until the defects are reduced. There is also disclosed a method comprising injection molding one or more parts, determining a mean injection pressure profile by measuring the injection pressure with the machine configured with a constant, desired injection velocity. Then the velocity profile is adjusted to reduce differences between the measured pressure and the mean pressure profile. A further method is disclosed wherein the kickback is calculated and adjusted from screw displacement, packing/holding time and pressure. Also disclosed is a method comprising injection molding one or more parts then determining the gate freeze time by incrementing the holding time and measuring the screw displacement.

The present invention relates to thermoplastic injection molding and inparticular to the automation of the die setter's role in the setting ofparameters of injection molding machines. The invention may also beapplicable to reactive injection molding.

Injection molding is one of the most important and efficientmanufacturing techniques for polymeric materials, with the capability tomass produce high value added products, such as the compact disc.Injection molding can be used for molding other materials, such asthermoset plastics, ceramics and metal powders. The process in itspresent form was developed in the mid 1950s, when the firstreciprocating screw machines became available. Material, machine andprocess variations are important in this complex multi-variable process.There are three interacting domains for research and development: 1)polymeric material technology: introduction of new and improvedmaterials; 2) machine technology: development of machine capability; and3) processing technology: analysis of the complex interactions ofmachine and process parameters. As improved product quality and enhancedengineering properties are required of polymeric materials, theinjection molding process has become increasingly complex: as serviceproperties increase material processability tends to decrease.

Thermoplastics can be classified as bulk or engineering materials.Engineering materials are typically more difficult to process, and moreexpensive, and therefore their processing would benefit the most fromautomated molding optimization (AMO). Injection molding is a batchoperation, so machine set-up ultimately affects productivity.

Any molding operation should aim to manufacture component products to aspecific quality level, in the shortest time, in a repeatable and fullyautomatic cycle. Injection molding machines usually provide velocitycontrol and pressure control, that is, control of the velocity of theinjection screw when filling the part and control of the pressureexerted by injection screw when packing/holding the part, respectively.The following description assumes the use of a modern injection moldingmachine, after circa 1980, with velocity control of the mold filling andpressure control of the packing/holding stages.

The typical injection molding cycle is as follows:

-   -   1) Plasticisation Stage: plasticisation occurs as the screw        rotates, pressure develops against the ‘closed-off’ nozzle and        the screw moves backwards (‘reciprocates’) to accumulate a fresh        shot (the molten polymer in front the screw), ready for        injection of melt in front of the screw tip. Back pressure        determines the amount of work done on the polymer melt during        plasticisation. Polymer melt is forced through the screw        non-return valve. Material is fed to the screw by gravity from a        hopper. The polymeric material may require conditioning,        especially in the case of engineering thermoplastics, to ensure        melt homogeneity and therefore that the melt has consistent flow        characteristics.    -   2) Injection/Filling Stage: the empty mold is closed, and a        ‘shot’ of polymer melt is ready in the injection unit, in front        of the screw. Injection/filling occurs, polymer melt is forced        though the nozzle, runner, gate and into the mold cavity. The        screw non-return valve closes and prevents back-flow of polymer        melt. In this, the mold filling part of the injection molding        cycle, high pressures of the order of 100 MPa are often required        to achieve the required injection velocity.    -   3) Packing/Compression Stage: a packing pressure occurs at a        specified VP or ‘switch-over’ point. This is the velocity        control to pressure control transfer point, i.e. the point at        which the injection molding machine switches from velocity        control to pressure control. ‘Switch-over’ should preferably        occur when the mold cavity is approximately full, to promote        efficient packing. The switch-over from injection to packing is        typically initiated by screw position. Switch-over can be        initiated by pressure, i.e. hydraulic, nozzle melt injection        pressures or cavity melt pressure parameters measured from the        machine. The end of this stage is referred to as ‘pack time’ or        ‘packing time’.    -   4) Holding Stage: a second stage pressure occurs after the        initial packing pressure and is necessary during the early        stages of the cooling of the molded part to counteract polymer        contraction. It is required until the mold gate freezes; the        injection pressure can then be released. This phase compensates        for material shrinkage, by forcing more material into the mold.        Typical industrial machine settings use one secondary pressure,        combining the packing and holding phases, to allow for easier        machine set-up. It has been shown that under packing results in        premature shrinkage, which may lead to dimensional variation and        sink marks. Over packing may cause premature opening of the tool        (i.e. the die or mold of the component(s) to be manufactured) in        a phenomenon known as flashing, difficulties in part removal        (sticking) and excessive residual stresses resulting in warpage.        Analysis of the packing phase is therefore an essential step in        predicting the final product quality. The portion of filling        after switch-over can be more important than the velocity        controlled primary injection stage. The end of this stage is        known as ‘hold time’ or ‘holding time’.    -   5) Cooling Stage: This phase starts as soon as the polymer melt        is injected into the cavity. The polymer melt begins to solidify        when in contact with the cavity surface. Estimating cooling time        is becoming increasingly important, especially when large        numbers of components are being molded. In order to calculate        cooling time, component ejection temperature should be known.        Cooling an injection molded product uniformly may mean cooling        the mold at different rates, in different areas. The aim is to        cool the product as quickly as possible, while ensuring that        faults such as poor surface appearance and changes in physical        properties are not encountered. The aims for a cooling system        are: (i) minimum cooling time, (ii) even cooling on part        surfaces, and (iii) balanced cooling between a core and a cavity        part of a two-plate tool system. Tool temperature control is        required to maintain a temperature differential AT between the        tool and the polymer melt. For example, a typical        polyoxymethylene melt temperature is 215° C., tool temperature        is 70° C., and hence ΔT=145° C. Adverse effects to product        quality must be expected for no or poor temperature control. The        cooling phase enables the polymer melt to solidify in the        impression, owing to the heat transfer from the molded product        to the tool. The tool temperature influences the rate at which        heat is transferred from the polymer melt to the tool. The        differences in heat transfer rate influence polymer melt        shrinkage, which in turn influences product density. This effect        influences product weight, dimensions, micro-structure and        surface finish. The tool cavity surface temperature is critical        to the processing and quality of injection molded components.        Each part of the product should be cooled at the same rate,        which often means that non-uniform cooling must be applied to        the tool. Thus, for example, cool water should be fed into the        inner parts of the tool cooling system (particularly in the area        of the gate) and warmer water should be fed into the outer        parts. This technique is essential when molding flat components        to close tolerances, or large components that include long melt        flow lengths from the gating position. Tool design must thus        preferably incorporate adequate temperature control zones (flow        ways), to provide the desired tool temperature. Tool temperature        control zones commonly use water for temperatures up to 100° C.,        above which oil or electrical heating is used.

Injection molding is one of the most sophisticated polymer processingoperations, with machine costs typically ranging from US$50,000 to wellover US$1,000,000 and tool costs ranging from $10,000 to well over$100,000. The vital operation of tool set-up is often not given theattention it deserves. If a machine is poorly set-up, then this willaffect the cost of production, through cycle time and part rejectionrates. Machine set-up is still regarded as a black art, reliant on theexperience of a manual die setter (i.e. the person responsible forsetting parameters on the injection molding machine to achieveacceptable quality production). In a typical injection moldingmanufacturing facility machine set-up is often overlooked with therequirement to ‘get parts out the door’. In this rush machine set-up isoften done with inconsistent strategies as different die setters havetheir own personal views as to what constitutes an optimal set-up.Manufacturing facilities typically have a high staff turn-over on theshop floor, and so training and maintaining an adequate level ofexperience is also a high cost.

An object of the present invention is to provide substantially automatedoptimization of at least a part of the injection molding set-up process.It is a further object of the present invention to provide moreconsistent machine set-up in an automated manner throughout amanufacturing facility.

According, therefore, to the present invention there is provided amethod for the automated optimization of an injection molding machineset-up process, said machine for manufacturing injection molded parts,including the steps of:

-   -   (1) manufacturing one or more parts with said machine;    -   (2) inspecting said parts for defects;    -   (3) reducing injection stroke in response to flashing or        increasing injection stroke in response to short shots; and    -   (4) reducing injection velocity in response to flashing or        increasing injection velocity in response to short shots,        wherein either step (4) is employed after step (3) if step (3)        is found to have substantially no effect or substantially no        further effect, or step (3) is employed after step (4) if        step (4) is found to have substantially no effect or        substantially no further effect, thereby reducing said defects.

Thus, if a machine setter observes that flashing or short shots are noteliminated by altering the injection stroke (or velocity), the set-upprocess may be improved by altering the injection velocity (or stroke).

The second invention also provides a method for the automatedoptimization of an injection molding machine set-up process, saidmachine for manufacturing injection molded parts and including aninjection screw and a configurable injection velocity, including thesteps of:

-   -   (1) manufacturing one of more parts with said machine;    -   (2) determining an injection pressure profile by measuring        injection pressure as a function of elapsed injection time with        said machine configured with a substantially constant, desired        injection velocity;    -   (3) measuring injection velocity as a function of elapsed        injection time and determining a profile of said measured        injection velocity;    -   (4) defining a mean pressure profile from said pressure profile        in a regime of substantially constant measured injection        velocity profile;    -   (5) adjusting said velocity profile over at least a portion of        an injection velocity phase in response to said pressure profile        to reduce differences between said pressure profile and said        mean pressure profile, thereby tending to lessen irregularities        in said pressure profile.

Preferably step (5) is performed only in said regime.

Preferably steps (1) and (2) are repeated a plurality of times to obtaina plurality of measurements of injection pressure profile and saidinjection pressure profile is determined from a mean of saidmeasurements.

Preferably steps (1) to (5) are repeated a plurality of times, therebyprogressively refining said velocity profile.

Thus, the velocity profile can be progressively adjusted to reduce oreliminate irregularities in the pressure profile. The step of adjustingthe velocity profile may be repeated to further reduce suchirregularities, to whatever tolerance is required.

1Preferably step (5) comprises increasing said injection velocity wheresaid pressure profile is less than said mean pressure profile, anddecreasing said injection velocity where said pressure profile isgreater than said mean pressure profile.

Preferably said mean pressure profile is linear.

Preferably said pressure profile is in the form of a derivative pressureprofile, obtained by differentiating said pressure profile with respectto time.

Thus, the method is preferably performed with the time derivative of thepressure, rather than the pressure itself.

Preferably said method includes determining a relationship between theinjection velocity and said pressure profile by perturbing saidinjection velocity about a predetermined velocity.

Preferably said relationship includes compensation for melt viscositychanges.

Preferably said viscosity changes include viscosity changes owing tomelt pressure and temperature changes.

Thus, the response of the pressure profile to changes to the injectionvelocity can be determined by performing test injections over a narrowrange of injection velocities.

Preferably the perturbation of said injection velocity is bypredetermined amounts, and more preferably the perturbation of saidinjection velocity is by ±10% and/or ±20%.

Preferably said pressure profile is derived from hydraulic injectionpressure. Alternatively said pressure profile is derived from melt flowpressure.

Preferably the method includes determining a viscosity model byperforming a material test of the injection melt material.

Thus, for non-Newtonian plastics (in reality all plastics) theprediction of the response of the pressure profile to changes in thevelocity profile can be improved if the viscosity is first measured.

The present invention further provides a method for the automatedoptimization of an injection molding machine set-up process, saidmachine for manufacturing injection molded parts and including aninjection screw and a configurable injection velocity, said screw havinga displacement, including the steps of:

-   -   (1) manufacturing one or more parts with said machine;    -   (2) defining as a first pressure the end of velocity control        phase pressure and as a second pressure the holding time        pressure;    -   (3) defining a linear relationship between packing/holding        pressure and time consistent with said first pressure and said        second pressure, between said first pressure and said second        pressure;    -   (4) defining said packing time as a time of maximum difference        between measured melt pressure and said linear relationship, or        as the switchover point if measured melt pressure increases        after the switchover point;    -   (5) determining a first screw displacement being the minimum        displacement of said screw before said packing time within a        packing/holding phase and a second screw displacement being the        displacement of said screw at said packing time; and    -   (6) calculating said kickback from the difference between said        first and second screw displacements, thereby allowing a        determination of said kickback from measurements of said screw        displacement at packing time.

Thus, maximum kickback-or the negative or backward movement of the screwat the velocity to pressure transfer point-may be determined from thescrew displacement at packing time.

The present invention still further provides a method for the automatedoptimization of an injection molding machine set-up process, saidmachine including an injection screw, including the steps of:

-   -   (1) setting an initial packing/holding pressure to a default low        pressure;    -   (2) performing at least a partial injection cycle;    -   (3) determining kickback from changes in screw displacement        during said at least partial injection cycle;    -   (4) incrementing said initial packing/holding pressure; and    -   (5) repeating steps (3) and (4) if kickback is unacceptably high        until kickback is reduced to a predetermined acceptable level,        or initial packing/holding pressure reaches maximum machine        pressure.

Preferably the initial packing/holding pressure is between 5% and 25% ofend of velocity control phase pressure, and a substantially uniformpacking pressure is used, and more preferably the initialpacking/holding pressure is approximately 10% of end of velocity controlphase pressure.

Preferably the initial packing/holding pressure is incremented bybetween 2% and 25% of said end of velocity control phase pressure, andmore preferably the initial packing/holding pressure is incremented byapproximately 5% of said end of velocity control phase pressure.

In one preferred embodiment, the method includes measuring kickback fora plurality of initial packing/holding pressures, predicting an optimuminitial packing/holding pressure from said measurements to minimizekickback, and incrementing said initial packing/holding pressure to saidoptimum initial packing/holding pressure.

In another aspect the present invention provides a method for theautomated optimization of an injection molding machine set-up process,said machine for manufacturing injection molded parts and including aninjection screw, including the steps of:

-   -   (1) defining a holding time equal to a predetermined default        value;    -   (2) performing at least a partial injection cycle;    -   (3) measuring a pressure stroke being the change in displacement        of said screw between packing time and said holding time;    -   (4) incrementing said holding time;    -   (5) repeating steps (3) and (4) until said pressure stroke        stabilizes or a part so produced is acceptable;    -   (6) defining a linear relationship between screw displacement        and time consistent with screw displacement at said packing time        and at said holding time, between said packing time and said        holding time;    -   (7) defining a gate freeze time as a time of maximum difference        between said screw displacement and said linear relationship,        thereby providing a value for said gate freeze time from        measurements of said screw displacement.

Preferably the method includes the additional steps of:

-   -   (8) repeating steps (6) and (7), and defining an initial        solidification time between said packing time and said gate        freeze time;    -   (9) repeating steps (6) and (7), and defining an intermediate        solidification time between said packing time and said initial        solidification time; and    -   (10) determining an intermediate pressure from the ratio of the        screw displacements at said intermediate time and at said gate        freeze time, referenced to said packing time.

Preferably the value of said holding time employed in step (6) isgreater than that defined in step (1) by a factor of between 1 and 3.

Preferably said predetermined default value is the greater of 2 timesinjection time and one second.

Preferably said stabilization occurs when said pressure stroke changesby less than a predetermined tolerance between successive measurements.

Preferably said holding time is incremented in step (4) by between 5%and 50%, and more preferably by approximately 20%.

Preferably said predetermined tolerance is between 2% and 10%, and morepreferably approximately 5%.

In one embodiment the present invention provides a method for theautomated optimization of an injection molding machine set-up process,said machine for manufacturing injection molded parts and including aninjection screw and a configurable injection velocity, including thesteps of:

-   -   (1) determining an optimum fill including:        -   (i) manufacturing one or more parts with said machine;        -   (ii) inspecting said parts for defects;        -   (iii) reducing injection stroke in response to flashing or            increasing injection stroke in response to short shots; and        -   (iv) reducing injection velocity in response to flashing or            increasing injection velocity in response to short shots,            wherein either step        -   (iv) is employed after step (iii) if step (iii) is found to            have substantially no effect or substantially no further            effect, or step (iii) is employed after step (iv) if            step (iv) is found to have substantially no effect or            substantially no further effect, thereby reducing said            defects;    -   (2) determining an optimum injection velocity profile,        including:        -   (i) manufacturing one of more parts with said machine;        -   (ii) determining an injection pressure profile by measuring            injection pressure as a function of elapsed injection time            with said machine configured with a substantially constant,            desired injection velocity;        -   (iii) measuring injection velocity as a function of elapsed            injection time and determining a profile of said measured            injection velocity;        -   (iv) defining a mean pressure profile from said pressure            profile in a regime of substantially constant measured            injection velocity profile;        -   (v) adjusting said velocity profile over at least a portion            of an injection velocity phase in response to said pressure            profile to reduce differences between said pressure profile            and said mean pressure profile, thereby tending to lessen            irregularities in said pressure profile.    -   (3) modifying a post-velocity control phase intermediate set-up        obtained after steps (1) and (2) in response to quality defects        detected in said parts manufactured with said intermediate        set-up to reduce said defects;    -   (4) a method of reducing kickback to an acceptable level to        determine a critical packing/holding pressure, including:        -   (i) setting an initial packing/holding pressure to a default            low pressure;        -   (ii) performing at least a partial injection cycle;        -   (iii) determining kickback from changes in screw            displacement during said at least partial injection cycle;        -   (iv) incrementing said initial packing/holding pressure; and        -   (v) repeating steps (iii) and (iv) if kickback is            unacceptably high until kickback is reduced to a            predetermined acceptable level, or initial packing/holding            pressure reaches maximum machine pressure.    -   (5) deducing material solidification time from measurements of        screw displacement to determine an optimal packing/holding        pressure profile, including:        -   (i) defining a holding time equal to a predetermined default            value;        -   (ii) performing at least a partial injection cycle;        -   (iii) measuring a pressure stroke being the change in            displacement of said screw between packing time and said            holding time;        -   (iv) incrementing said holding time;        -   (v) repeating steps (iii) and (iv) until said pressure            stroke stabilizes or a part so produced is acceptable;        -   (vi) defining a linear relationship between screw            displacement and time consistent with screw displacement at            said packing time and at said holding time, between said            packing time and said holding time;        -   (vii) defining a gate freeze time as a time of maximum            difference between said screw displacement and said linear            relationship, thereby providing a value for said gate freeze            time from measurements of said screw displacement;    -   (6) modifying a post-pressure control phase preliminary set-up        obtained after (1) to (5) in response to defects detected in        said parts manufactured with said preliminary set-up to reduce        said defects.

Preferably step (iii) of step (4) includes determining kickback frommeasurements of said screw displacement at packing time, including thesteps of:

-   -   (a) manufacturing one or more parts with said machine;    -   (b) defining as a first pressure the end of velocity control        phase pressure and as a second pressure the holding time        pressure;    -   (c) defining a linear relationship between packing/holding        pressure and time consistent with said first pressure and said        second pressure, between said first pressure and said second        pressure;    -   (d) defining said packing time as a time of maximum difference        between measured melt pressure and said linear relationship, or        as the switchover point if measured melt pressure increases        after the switchover point;    -   (e) determining a first screw displacement being the minimum        displacement of said screw before said packing time within a        packing/holding phase and a second screw displacement being the        displacement of said screw at said packing time; and    -   (f) calculating said kickback from the difference between said        first and second screw displacements, thereby allowing a        determination of said kickback from measurements of said screw        displacement at packing time.

Preferably step (5) includes the additional steps of:

-   -   (viii) repeating steps (vi) and (vii), and defining an initial        solidification time between said packing time and said gate        freeze time;    -   (ix) repeating steps (vi) and (vii), and defining an        intermediate solidification time between said packing time and        said initial solidification time; and    -   (x) determining an intermediate pressure from the ratio of the        screw displacements at said intermediate time and at said gate        freeze time, referenced to said packing time.

In each of the above aspects of the present invention, the methodpreferably includes:

-   -   determining said machine's velocity control response time, and    -   employing time steps equal to of greater than said response        time.

Preferably said time steps are greater than 1.5 times said responsetime, and more preferably equal to 2 times said response time.

In the above aspects of the present invention, nozzle melt pressure,injection cylinder hydraulic pressure, forward propelling force appliedto said screw, or any other measure proportional to or equal to saidnozzle melt pressure may be used as a measure of, in place of, or todetermine, injection pressure.

Preferably said injection cylinder hydraulic pressure is used as ameasure of or to determine said injection pressure.

In order that the invention may be more clearly ascertained, preferredembodiments will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic representation of the automated machineoptimization method according to a preferred embodiment of the presentinvention;

FIG. 2 is a graph illustrating schematically the influence of velocityand velocity stroke on the filling process; and

FIG. 3 depicts a typical pressure profile resulting from a pressureprofiling method according to a preferred embodiment of the presentinvention.

The present invention (referred as to as Automated Molding Optimizationor AMO) is used in the setting up the injection/filling velocity andpacking/holding pressure profiles. Other injection molding machineparameters, including barrel temperatures, mold temperatures, coolingtime and screw rotational velocity are presently the responsibility ofthe die setter.

The fundamental principle of AMO's velocity optimization is to profileregarding an inferred mold geometry, derived from the pressuredifferential. Pressure phase optimization is used to profile regardingan inferred polymer solidification, derived for a precise measurement ofscrew displacement. AMO determines machine and material characteristicsin-line from the machine without the need for user interaction,resulting in optimized profiles that are ‘in-phase’ with the machinedynamics, material and mold geometry.

FIG. 1 is a flow chart summarizing the role of the AMO method accordingto a preferred embodiment. In FIG. 1, the various inputs are ComputerAided Engineering (CAE) model 10, Machine Information 12, MaterialInformation 14, Processing Conditions 16 a and 16 b, and Estimates ofVelocity and Velocity Stroke 18. The inputs are employed in anoptimization stage (MF/OPTIM or “Moldflow optimization”). Feedback onthe design of the part is indicated with a dashed line 20.

The preferred embodiment AMO method has six process optimization phases:

-   -   1. Velocity and velocity stroke, based on a single-step constant        velocity;    -   2. Injection/Filling Velocity profiling;    -   3. Velocity defect elimination;    -   4. Packing pressure magnitude determination;    -   5. Gate freeze determination and pressure profiling;    -   6. Pressure phase defect elimination.

In general, if the screw gets too close to bottoming out, the screwcharge profile is shifted back. This takes two shots, since the firstmay not plasticate to the new position. If the cycle time is too longAMO will ignore the cycle.

These six phases are summarized as follows:

-   1) Determination of velocity stroke and velocity settings: This    phase assumes that a substantially uniform velocity profile is used,    and that the tool can be adequately filled using such a profile. The    rules used within this phase converge on settings that produce a    ‘good part’, if a poor estimate of the velocity stroke or volume is    input. A ‘critical fill’ velocity stroke is determined, to ensure    that no packing occurs during the velocity controlled injection    stage. The critical fill is the point at which the part is only just    filled. Sometimes the polymer within the cavity is overfilled, but    does not show any visible defects. The initial velocity profile is    generated from:    -   i) an estimate of the velocity stroke, entered directly or as a        part volume, and ii) velocity, typically 50% of the machine's        maximum capability. The charge stroke is initially set equal to        approximately 1.1×velocity stroke. This phase requires user        feedback after each part manufactured. At this stage, other        velocity related and pressure phase related defects are ignored.-   2) The first procedure in this phase is to determine an estimate of    the relationship between injection velocity and the mean    differential of the nozzle melt pressure profile. The nozzle melt    pressure may be derived from hydraulic injection pressure multiplied    by a screw intensification ratio. The injection velocity is    perturbed about the velocity from phase 1, by predefined    percentages, for example ±10%, ±20%. The next phase is to determine    the nozzle pressure profile, for stable processing conditions,    obtained using a uniform velocity profile, and then differentiate    the profile. Machine response time is determined from the velocity    profile. Using the pressure differential information during the    velocity stage an optimized velocity profile is obtained. The    profile is generated in two stages runner and cavity, and combined    using a response check.-   3) This phase involves velocity related defect elimination. The main    objective is to vary the velocity profile to achieve a part with no    velocity related defects. Velocity related defects are corrected.    Defects include jetting, delamination, gloss marks, burn marks, weld    lines, flash etc. Comment: The user simply selects the defect. In    the case of conflicting defects, it is required to converge on a    compromise point. One part (good quality immediately) is the    minizmim, the maximum depends on the user's assessment. Three parts    is often typical.-   4) This phase determines a critical packing pressure, i.e. a    pressure level that will help to eliminate back flow of material,    out of the cavity. The approach is to start low and increase the    pressure until the desired level is reached.-   5) This phase determines an inferred gate freeze, initial    solidification and intermediate times. The times are determined by    precisely monitoring the screw movement with a uniform pressure    profile applied. Gate freeze time and initial solidification time is    found, and the packing/holding profile is generated. This process    does not require the weighing of any molded parts. We infer the    cavity pressure from non-cavity sensors, specifically hydraulic    pressure and screw movement.-   6) This phase involves pressure related defect elimination. The main    objective is to vary the pressure profile to achieve a part with no    pressure related defects. Pressure related defects are assessed.    These are flash, sink, warpage, and dimensional tolerance (too    large/too small).

Phases 1 to 3 are initiated with zero or a very low packing pressure,typically only for 1 second.

These six phases are described in more detail as follows.

Phase 1

This phase comprises the determination of velocity stroke and velocitysettings. A constant velocity profile that results in a full part isfound. All defects (apart from flash and short shot) are ignored.

The pressure profile is initially set to substantially zero.

Phase 1.1: User Estimation

The user is asked to provide an estimate of the part volume. The volumeshould be easily obtained from the die maker. The volume is divided bythe area of the screw to give a velocity stroke; alternatively, the diesetter can estimate the velocity stroke directly. An accurate estimateof part volume may also be obtained from a Computer Aided Engineering(CAE) model.

The estimated velocity stroke is compared with the maximum stroke of themachine to ensure the machine is a reasonable size for the part beingmade. The following checks are made:

-   -   charge stroke>maximum stroke    -   velocity stroke>90% maximum stroke    -   velocity stroke<5% maximum stroke

The user also estimates the screw velocity. The velocity could beestimated by a 2D flow analysis, but at present this is seen asunwarranted, as the user would have to enter more information (e.g.material information, length of dominant flow path). Further, the usercan be expected to have a reasonable idea of the correct velocity to usefrom their experience.

A flat filling profile is generated from these estimates; the VP pointis configurable as a percentage of the estimated velocity stroke(default is 20%).

Phase 1.2: Optimization of Estimation

This phase aims to refine the user's estimate of the stroke so that afull (not flashed or short) part is made. Throughout the steps belowconfigurable adjustment parameters are used. After each change to theset points a configurable number of parts are made to try to ensuresteady state conditions.

The method of this phase was developed from the discovery of arelationship between injection velocity and velocity stroke, and theoptimization of the material fill. This relationship is depictedschematically in FIG. 2.

The following steps summarize this phase:

-   1. A part is made, and feedback about the part quality is requested    from the user.-   2. If the part is short, the stroke is increased by moving the VP    changeover point.-   3. If the part is flashed, the stroke is decreased by moving the VP    changeover point.-   4. If the part is both short and flashed, the user is asked for more    feedback: if the user thinks that there is melt freeze-off, the    velocity is increased and the stroke reduced, otherwise the opposite    occurs.-   5. If the part is full, this phase is complete.-   6. A part is made with the new set-points, but this time the user    has the opportunity to specify that no improvement occurred. If the    user specifies ‘No Improvement’, the following steps 7 to 9 are    followed.-   7. If the previous response was ‘short’, then velocity and stroke    are increased. This allows for the short to have been caused by melt    freeze off.-   8. If the previous response was flash, then velocity and stroke are    decreased-   9. If the previous response was flash and short, the velocity is    decreased and the stroke increased. The changes are made twice to    make up for the previous (now known to be incorrect) modifications.-   10. If the user does not specify ‘No improvement’, but instead    repeats the previous quality assessment, then the previous set-point    modifications are repeated.-   11. If the user specifies short shot when previously specifying    flash (or vice versa), the adjustment factor is halved to allow the    set-points to converge. A configurable minimum adjustment factor is    used to prevent adjustments becoming insignificant.-   12. If velocity stroke increases cause the VP changeover point to be    less than a configurable percentage of the velocity stroke, the    charge stroke is increased before the next part is made.-   13. When the charge stroke is increased, the next part is ignored,    since the injection molding machine may have finished plasticating    to the now incorrect position.-   14. If no improvement is selected on three consecutive occasions,    the procedure halts and the user asked to modify melt/mold    temperatures.    Phase 1.3: Obtaining Critical Fill

After phase 1.2 is complete, a full part exists. However, the part maybe overfilled, which is often the cause of internal stresses. It willalso require an overly high packing/holding pressure to eliminatekickback. This phase attempts to eliminate this problem by obtaining astate of ‘critical fill’.

Firstly, the stroke is reduced, as though the user had indicated flash.This is repeated each time the user indicates a full part. Eventually, apoint is reached where the stroke is small enough to cause a short shotto occur. When the user indicates short shot, the stroke is increased(it should be noted that the change in stroke is smaller than previouslydue to convergence). When the part regains ‘fullness’, critical fill hasbeen achieved.

Phase 2: Injection/Filling Velocity Profiling

This stage puts ‘steps’ into the velocity profile. These steps helpmaintain a constant flow front velocity, which in turn minimizesinternal stresses in the molded part. Weightings are imposed on the rawvelocity profile found to ensure it slows at the end of fill, which isknown to improve burn marks, and at the runner (to prevent jetting).

This phase is employed after phase 1, and if the velocity profile is ofconstant velocity and pressure (nozzle or hydraulic) and displacementtransducer data are filtered and available.

It is assumed that the displacement at which inflection points in thepressure curve are located does not change significantly when thevelocity is altered.

Prior to calculating the velocity profile, the pressure information froma number of parts is stored and then averaged, in an attempt to smoothout deviations between cycles. A number of parts may also be ignoredbefore this averaging takes place to achieve steady state conditions;both the number of parts to average and the number to ignore areconfigurable, with defaults of 1 and 0 respectively.

Phase 2.1: Determination of Material Properties

If AMO is to profile the velocity control, then it is necessary to knowhow large to make the steps. Thus, it is necessary to determine therelationship between the velocity set-point and the magnitude of$\frac{\mathbb{d}p}{\mathbb{d}t}.$For example, if $\frac{\mathbb{d}p}{\mathbb{d}t}$must be increased by 10%, this relationship is required in order todetermine how high the velocity step should be.

The following steps are taken to determine the relationship betweenvelocity and $\frac{\mathbb{d}p}{\mathbb{d}t}:$

-   1. The percentage velocity deviations are read from the    configuration file;-   2. The velocity is altered, a part is made, and the mean magnitude    of the $\frac{\mathbb{d}p}{\mathbb{d}t}$-    response (during velocity control) is recorded;-   3. If more experiments are required, the velocity is altered    according to the next percentage in the configuration file, and step    2 is repeated. If not, the velocity is reset to the user's estimate,    and step 2 is repeated one last time.-   4. Linear regression is used to find an equation relating the mean    $\frac{\mathbb{d}p}{\mathbb{d}t}$-    values recorded to the velocity set-points used.    Phase 2.2: Determination of Displacement Induction Time

Recorded data before the induction time should be ignored, sinceessentially nothing is happening, so it is necessary to determine thedisplacement induction time, which is a measure of the time required forthe screw to commence movement after the data acquisition systemreceives an injection start signal.

The displacement induction time is found when the displacement dataindicates the screw has moved beyond a small threshold distance. Thethreshold is calculated as a percentage of the charge stroke (e.g.0.1%); this threshold should be typical of the noise level ofdisplacement transducers.

Phase 2.3: Determination of Pressure Induction Time

Similarly, the pressure induction time is a measure of how long it takespressure to begin increasing after the data acquisition system receivesan injection start signal. This may be longer than the displacementinduction time if decompression is used at the end of plasticisation.

The pressure induction time is found when the pressure data indicatesthe screw has increased above a certain small threshold above theinitial pressure (this allows for transducer zero error). The thresholdis calculated as the minimum of a percentage (e.g. 0.1%) of the maximummachine pressure and an absolute pressure value (e.g. 0.1 MPa). Thisthreshold approximates the noise level on pressure transducers.

Phase 2.4: Determination of Machine Response Time

The injection molding machine cannot follow steps in the velocityprofile if the steps are too short. This minimum time is defined interms of the machine response time. Hence, it is necessary to determinethe machine response time, which is a measure of the time required bythe screw to obtain a given velocity.

The response time is simply the time at which the velocity data exceeds85% of the target velocity.

Phase 2.5: Determination of Pressure Derivative (wrt Time)

As discussed above, it is desirable to keep the flow front velocityreasonably constant by introducing steps into the velocity profile. Thesize and location of these steps is based upon the$\frac{\mathbb{d}p}{\mathbb{d}t}$calculations. The quantity $\frac{\mathbb{d}p}{\mathbb{d}t}$provides an indication of the part geometry as seen by the advancingflow front. When $\frac{\mathbb{d}p}{\mathbb{d}t}$increases, the flow front is faced with a narrowing in thecross-sectional area of the cavity.

A 33 point Savitsky-Golay smoothing filter is used to smooth thepressure information. The square root of all pressure information istaken. This allows for large $\frac{\mathbb{d}p}{\mathbb{d}t}$values increasing at much faster rate when velocity is increased thanaverage $\frac{\mathbb{d}p}{\mathbb{d}t}$values. It should be noted that in Phase 1 there is calculated a linearrelationship between mean $\frac{\mathbb{d}p}{\mathbb{d}t}$and the velocity set-point. The quantity$\frac{\mathbb{d}p}{\mathbb{d}t}$is calculated by subtracting the next pressure value by the currentpressure value, and dividing by the sampling period.Phase 2.6: Determination of Gate Time

Knowledge of when the flow front reaches the gate allows the method tohave separate velocity profile steps for the runner system. The ‘gatetime’ is thus the time at which the flow front reaches the gate.

The gate time is taken as the maximum of the three calculations detailedbelow. The maximum is used to attempt to ensure that a point away fromthe initial $\frac{\mathbb{d}p}{\mathbb{d}t}$‘hump’ is found.

-   -   1) $\frac{\mathbb{d}p}{\mathbb{d}t}$    -    ‘zero time’: Between the induction time and 50% of the        injection time, $\frac{\mathbb{d}p}{\mathbb{d}t}$    -    is checked to see when it falls below zero. The gate time is        the point at which it rises back above zero;    -   2) $\frac{\mathbb{d}p}{\mathbb{d}t}$    -    ‘low time’: the maximum $\frac{\mathbb{d}p}{\mathbb{d}t}$    -    between the induction time and 50% of the fill time is found.        The mean $\frac{\mathbb{d}p}{\mathbb{d}t}$    -    between the time at which this maximum occurs and the end of        the fill time is found. Where $\frac{\mathbb{d}p}{\mathbb{d}t}$    -    first falls below this mean is the gate time. Note that the low        time is always less than the zero time, so this calculation is        only made if $\frac{\mathbb{d}p}{\mathbb{d}t}$    -    never falls below zero; and    -   3) Velocity stabilization time: Between 70% of the fill time        back to the induction time, a moving average (over a three-point        window) of the velocity data is calculated. The gate time falls        where the moving average is outside (μ_(vel)±12σ_(vel)), where        μvel and σ_(vel) are calculated during an assumed steady state        portion of the velocity data (e.g. between 70% and 90% of        filling time). In other words, the method looks for the point at        which the velocity first becomes stable, with an upper limit of        70% of the filling time imposed.        Phase 2.7: Determination of Stepped dp/dt Profile

As discussed above, it is desirable to keep the flow front velocityreasonably constant by introducing steps into the velocity profile. Thesteps in the velocity profile should correspond to the cross-sectionalarea of the cavity, which in turn should have a strong relationship withthe stepped $\frac{\mathbb{d}p}{\mathbb{d}t}$profile. The stepped $\frac{\mathbb{d}p}{\mathbb{d}t}$profile approximates the $\frac{\mathbb{d}p}{\mathbb{d}t}$calculations (after the gate time) as a series of steps. The number ofsteps is limited by a configurable limit, and the size of the steps neednot depend on the machine response time.

The maximum of $\frac{\mathbb{d}p}{\mathbb{d}t}$between the gate time and the end of filling is found. A configurablepercentage (e.g. 10%) of the maximum $\frac{\mathbb{d}p}{\mathbb{d}t}$value Δ is calculated. Step number n is initialized to 0, and data countindices i and k to the induction time and zero, respectively. Index i isused to store the start position of each step in the$\frac{\mathbb{d}p}{\mathbb{d}t}$data, and k is used to iterate through the data within each step. Aninitial $\frac{\mathbb{d}p}{\mathbb{d}t}$value sum is stored for time=i+k. If${{{{{sum}\text{/}k} - {\frac{\mathbb{d}p}{\mathbb{d}t}\left\lbrack {i + k + 1} \right\rbrack}}} > \Delta},$then the profile step n is set equal to sum/k, n is incremented, i setto i+k, and the method returns to phase 2.4. Otherwise, sum is increasedby${\frac{\mathbb{d}p}{\mathbb{d}t}\left\lbrack {i + k + 1} \right\rbrack},$k is incremented, and the method returns to the start of this phase(2.7) unless k=fill time. The method reaches this stage when k=filltime. The final profile step=sum/k, and any negative profile steps areset to zero.Phase 2.8: Determination of Stepped Velocity Profile

Stepped velocity profiles can be entered into machine controllers asset-points, and should try to maintain a constant flow front velocity asthe polymer moves into the cavity. The velocity profile determined inthis section is based on the stepped $\frac{\mathbb{d}p}{\mathbb{d}t}$profile determined by the previous phase, and does not take into accountmachine response time.

From the stepped $\frac{\mathbb{d}p}{\mathbb{d}t}$pressure profile, the following parameters are calculated:$\begin{matrix}{1.\quad{Mean}{\quad\quad}\frac{\mathbb{d}p}{\mathbb{d}t}} \\{2.\quad{Maximum}\quad\frac{\mathbb{d}p}{\mathbb{d}t}} \\{3.\quad{Minimum}\quad\frac{\mathbb{d}p}{\mathbb{d}t}}\end{matrix}$

-   4. For each step n in the $\frac{\mathbb{d}p}{\mathbb{d}t}$-    profile, the corresponding velocity step, where:    ${velocity}_{n} = {\left( {{{mean}\quad\frac{\mathbb{d}p}{\mathbb{d}t}} - \frac{\mathbb{d}p}{\mathbb{d}t^{n}}} \right)/\left( {{\max\frac{\mathbb{d}p}{\mathbb{d}t}} - {\min\frac{\mathbb{d}p}{\mathbb{d}t}}} \right)}$

This gives the velocity profile scaled about 1, where 1 is the meanvelocity (the user's estimate).

Phase 2.9: Determination of Runner Velocity

The runner velocity is the first step in the velocity profile. Therunner velocity is chosen using the ratio of the maximum$\frac{\mathbb{d}p}{\mathbb{d}t}$between the induction time and the gate time, and the mean pressure ofthe stepped pressure profile (see Phase 2.7: Determination of Steppeddp/dt Profile). As the ratio increases, the runner velocity decreases;the ratio is limited so that the runner velocity is never less than themean velocity after the gate.${{Runner}\quad{velocity}} = {1 - {0.1 \times \left( {\max{\frac{\mathbb{d}p}{\mathbb{d}t}/{mean}}\quad{of}\quad{stepped}\quad{pressure}\quad{profile}} \right)}}$Phase 2.10 Determination of End of Fill Velocity

A standard die setters' heuristic is to slow the velocity toward the endof fill. This helps prevent air becoming trapped within the cavity, andtherefore helps prevent burn marks. It also helps ensure the part is notoverfilled, and allows for a smoother transition into thepacking/holding phase. The end of fill velocity is the last step in thevelocity profile. The default is the last 10% of fill, though this isconfigurable.

A ratio of $\frac{\mathbb{d}p}{\mathbb{d}t}$during the end of fill segment compared with$\frac{\mathbb{d}p}{\mathbb{d}t}$in the 10% of fill immediately prior is calculated. If this ratio ishigh, the velocity at end of fill will be low, but limited to 50% of theprior velocity. If the ratio is low (i.e.$\frac{\mathbb{d}p}{\mathbb{d}t}$decreases at end of fill) the last velocity step is limited to theimmediately preceding velocity, i.e. the velocity is not increased atend of fill.Phase 2.11 Compensating for Response Time

The stepped velocity profile determined in the previous phase assumesthe machine has infinitely fast response to changes in the set point. Ofcourse, this is not realistic, and so steps should be lengthened to takethe actual response time into account. Steps close together in magnitudeare merged since the difference is likely to be overwhelmed by the errorin the controller. If such small differences were left in the velocityprofile the algorithm would lose credibility. A maximum number of stepsare specified since nearly all IMM controllers on the market today arelimited in this way.

This phase lengthens the step size of the velocity profile calculated inthe previous phase if they are less than the response time calculated inPhase 2.4: Determination of Machine Response Time. Furthermore, stepsthat are closer together in magnitude than the desired threshold aremerged. If at the end of this process there are more steps than allowed,this process is repeated with a larger response time and a largerthreshold.

Each step in the velocity profile is merged with the next step, if thelength of the step is less than the response time. The steps are mergeduntil the merged step length is greater than or equal to the responsetime. The resulting step has a velocity corresponding to the weightedvelocity of the two steps. For example:newVelocity=(time1×velocity1+time2×velocity2)/(time1+time2)

This process is repeated until all steps have been checked for responsetime.

If the duration of the last step is too short, it is merged with thesecond last step. The profile is rescaled to the previous maximum andminimum. This resealing may be limited by a configuration file parameterso that small steps are not blown out of proportion. The resealing alsomaintains 1 (the user's estimate) as the mean value. The magnitude ofeach velocity step is compared against the magnitude of the next step.If the difference is less than 10% of the maximum velocity, the stepsare merged as described above, and the profile rescaling is returned to.The number of steps in the profile is checked. If it is greater than themaximum number allowed, this stage is repeated with a response time 20%longer, and a velocity difference threshold of 20% instead of 10%.

Phase 2.12 Converting Time to Displacement, and Velocity to PhysicalUnits

Most injection molding machine controllers accept velocity profiles interms of screw displacement (rather than time). Also, the velocityvalues are currently normalized, and need to be scaled to physical units(e.g. mm/s) before they can be passed to an IMM controller.

A conversion factor, α, is calculated using the relationship found inPhase 1. For each velocity step n:velocity_(n)=user velocity estimate×((velocity_(n)−1)×α+1)

The result is in S.I. units (m/s).

To convert times to displacements, a conversion factor—between theset-point velocity stroke and the number of samples during filling—iscalculated. The conversion factor does not have to take into accountvelocity magnitudes earlier in the profile being different to those usedwhen the part was made, since the velocity step changes should berelative to the flow front position, not the time at which theyoccurred. Set the displacement of each step from the charge stroke usingthe conversion factor:displacement_(n) =charge stroke−conversion factor×step sample number_(n)Phase 3: Velocity Defect Elimination

At this point, the magnitude of the velocity steps is an arbitrarypercentage of the maximum velocity of the machine (although they shouldbe approximately correct relative to each other). As a result, moldingdefects could occur. This stage attempts to rectify the defects relatedto the velocity profile by executing heuristics in response to userfeedback.

There are two prerequisites: firstly that one part has been made withthe velocity profile from phase 2, and secondly that user feedback hasbeen supplied regarding the quality of the part produced. The feedbackis one of the following defects: no defect, flash, short shot, weld,burn, jetting, streak, gloss, delamination, and record grooves.

It is assumed that changing the average magnitude of the velocityset-point does not effect the position of inflection points in thepressure curve.

The following responses are made to each defect, in making anothermolding to ensure good quality.

-   -   1. Flash: Decrease all velocity steps by a multiplier.    -   2. Short: Increase all velocity steps by multiplier    -   3. Weld: Same as short.    -   4. Burn: The user is asked for more information; is the burn        mark near the gate, all over, or near the end of fill. If the        burn is all over, decrease all velocity steps. If the burn is        near the end of fill, reduce the velocity of the screw at all        points in the last 25% of the filling profile. Burn marks near        the gate are treated in a similar fashion, except the first 25%        of velocity points are altered.    -   5. Jetting: decrease all velocity points in the first 25% of the        velocity profile.    -   6. Streak marks: as for burn marks, except the user gets a        choice of ‘all over’ or ‘end of fill’.    -   7. Gloss marks: increase the entire velocity profile by a        multiplier.    -   8. Delamination: decrease the entire velocity profile by a        multiplier    -   9. Record Grooves: As for gloss marks.

The rule base fails if the desired action cannot be taken; in this eventthe user is informed of the situation and given advice on how to solveit (via on-line help).

Phase 4: Obtaining the Correct Packing Pressure

At this point, the injection molding machine is using a default lowpressure. The correct level of pressure to use during the pressurecontrol stage that avoids kickback is desired. This stage does this, butdoes not profile the pressure control set-points, or find the time thatpressure control should be maintained.

There are three prerequisites: firstly that Phase 3 has completedsuccessfully, secondly that the maximum packing pressure is known, andthirdly that steady state conditions prevail.

Phase 4.1: Initial Pressure Control Set-points & Velocity StrokeReduction

The pressure control time is set to twice the injection time (or 1 s,whichever is greater), the pressure level is 5% of the end of fillpressure, and a ‘rectangular’ shape pressure profile is used.

Further, to ensure the melt is not compressed during filling, thevelocity stroke is reduced by 2%, in line with current molding practice.

Phase 4.2: Determination of Kickback

Kickback is defined as the distance travelled by the screw in thereverse direction to injection during pressure control after the packingtime. This is caused by the pressure control set-point being less thanthe back pressure exerted by the melt in front of the screw.

It is desirable to eliminate kickback to avoid polymer flowing out ofthe cavity, which is known to be a cause of sink marks, warpage andother dimensional problems.

The maximum kickback displacement is found by finding the packing time.The kickback is then the distance from the minimum displacement beforethe packing time to the displacement at the packing time. If thekickback is not negative, it is set to zero.

The first task is to determine the packing time by examining the nozzlemelt pressure (or the hydraulic pressure). The equation of a straightline from the pressure at the v/p switchover point time to the pressureat the hold time is calculated, and then the time at the maximumdifference between the straight line and the recorded pressure curve isthe packing time.

However, a pressure increase after v/p switchover indicates that nokickback has occurred. In this case, the packing time is the v/pswitchover point. This does not mean that the packing time is always atthe v/p switchover point when no kickback occurs.

Phase 4.3: Kickback Elimination

This procedure is employed where kickback is greater than zero. If thereis no kickback, the pressure level is acceptable.

The initial packing/holding pressure is increased by 5% of the end ofvelocity control phase pressure (or ‘end of fill pressure’). Phase 4.2is then repeated until the difference between kickback for the currentshot and last shot is less than a configurable percentage, or until themaximum machine pressure is reached.

This procedure should not fail, as kickback will only occur if the fillpressure is significantly greater than the packing/holding pressure.Therefore, a suitable packing/holding pressure should be obtainable onthis machine.

Phase 5: Estimating Holding Time

The gate pressure control time is determined by means of an end pointfit between the ‘pack’ time and the ‘search time’ using data recorded upto the ‘hold time’.

Phase 5.1 Determination of Gate Freeze Time and Holding Time

To this point, the holding time has been taken to be twice the injectiontime. This is an arbitrary value, and in most cases is too short. Theaim of this stage, therefore, is to find a more accurate holding time,as short holding times can result in molding defects, such as sinkmarks, since the polymer will be able to flow back out of the cavitybefore solidification occurs. Further, although phase 5 estimates thegate freeze time, the procedure relies on the current holding time beinglonger than the gate freeze time. An arbitrarily long holding time cannot be used since there is a slight risk of tool damage.

The holding time is increased by 50% of its current value each shot,until the forward movement of the screw between the packing time andholding time converges. Convergence is defined as a change of less than5% in movement from one shot to the next. The current time is chosen(rather than the old time) to allow the gate freeze estimation to bemore accurate. Sometimes the screw movement will not converge for areasonable holding time, since there may be slippage on the check ringvalve or the polymer behind the gate (e.g. in the runner system) maycontinue to compress after the gate has frozen. To prevent the holdingtime increasing without limit, a maximum of 30 s is used.

Phase 5.2: Pressure Profiling

Pressure profiling is designed to find the initial solidification timeto and gate freeze time tf, and an intermediate time, ti, between thesetwo. Further, the desired pressure Pi at ti is calculated, while thepressure at tf is set to zero, since any pressure applied after gatefreeze time will have no effect on part quality after this time. FIG. 3shows the form of the resulting profile, where the point correspondingto ts is indicated at 30, Pi and ti at 32, tf at 34 and the pressurelevel determined in the previous stage at 36.

Two prerequisites are that the pressure level and the holding time havebeen determined.

Profiling the pressure control set-points helps prevent over packing ofthe part as the polymer in the cavity cools, since the pressure will beapplied to a smaller molten area as cooling progresses. The internalstress of the part may also be improved, since a more similar force willbe applied to each fraction of the cooling mass. The point at time tihelps to more accurately estimate the cooling rate, since it is unlikelyto be linear. The gate freeze time tf is determined using end point fitson the pressure and displacement data. An additional end point fitbetween the packing time and tf over the displacement data gives ts, anda final end point fit (again using displacement information) between tsand tf gives ti. Pi is determined from the following calculation:${Pi} = {{Porig}\left( \frac{{Dpacktime} - {Dintermediatetime}}{{Dpacktime} - {Dfreezetime}} \right)}$where

-   Dpacktime is the screw displacement at the packing time,-   Dintermediatetime is the screw displacement at ti,-   Dfreezetime is the screw displacement at tf, and-   Porig is the pressure found in Phase 4.

If the gate freeze time cannot be found, the original pressure controltime is used instead.

Once the packing time is established, the displacement curve is analyzedto determine the gate freeze time. The search time is greater than orequal to the holding time. It is determined by drawing a constantdisplacement line from the end of recorded data up to 3×(holdtime−packing time)+hold time, and also drawing a line extrapolated fromthe displacement curve between the 75% to 95% time locations (m_(d)).

The gradient of the resulting end point fit line (m_(E)) is thencompared to m_(d), and the search time is decreased until m_(E)>k×m_(d),where 1.3≦k≦3.5 and preferably k=2.

This technique allows a more accurate estimation of the gate freeze timewithout the actual holding time increasing.

Pack displacement is the distance moved by the ram after the packingtime, and the gate freeze time is the maximum difference between the endfit line and the recorded displacement curve.

Phase 6: Removing Packing/Holding Related Defects

After Phase 5 is finished, there is still some possibility of qualitydefects remaining. However, the defects present should not be related tothe velocity control (filling) phase, since these were eliminated inPhase 3. The defects that are related to the pressure control set-pointsare:

-   -   Flash    -   Warpage    -   Sink    -   Dimensional Tolerance

A simple rule base is used to eliminate the defects listed in theintroduction. The rule base does not alter the shape of the profile—itis simply ‘stretched and squeezed’. This rule base is:

-   -   Flash: Decrease the magnitude of the profile by 10%.    -   Warpage: Decrease the magnitude of the profile by 5%.    -   Sink: Increase the magnitude of the profile by 5%.    -   Also increase the pressure control time by 5%.    -   Dimensional Tolerance: If the part is too large, decrease the        magnitude of the profile by 5%. If the part is too small,        increase the magnitude by 5%.

In conclusion, AMO allows process optimization to be performed quicklyby molders. The process optimization is ‘in-phase’ with the actualprocess, i.e. it compensates for specific machine dependent parameters,such as leakage from the check-ring, poor velocity control, utilizingthe actual processing conditions.

Thus, AMO:

-   -   provides consistent machine set-up allowing operators with        little diesetting experience to optimize machine set-up;    -   reduces the requirement for skilled labour, i.e. de-skills the        set-up procedure;    -   provides process optimization throughout molding facilities;    -   provides better integration of mold design and part production,        with a continuation of Moldflow's commitment to bring the        benefits of simulation upstream into the world of the product        designer and to link simulation downstream into the production        environment; and    -   provides easier installation on modern velocity controlled        injection molding machines. Machine process information is        obtained from standard machine transducers.

AMO optimizes velocity and pressure phase profiles. Velocity profilingassists in eliminating flashing, short shots, splay mark/gateblush/molecular stripping, streak marks/flow lines,delamination/flaking, gloss/gloss bands, burning, jetting, sink marksand warpage. Velocity profiling also optimizes process repeatability,injection time and clamp force.

Pressure profiling assists in eliminating flashing, warpage, variation,sink marks and demolding. Pressure profiling optimizes criticaldimensions and back flow of polymer.

Thus, AMO allows machine operators with little previous diesettingexperience to set-up the injection molding machine in approximately 25to 40 cycles. AMO will help eliminate most molding problems without theneed for an experienced die setter. It automates the machine set-upprocedure by determining optimum processing conditions by theintelligent interpretation of in-line process measurements.

Modifications may be made to the invention as will be apparent to aperson skilled in the art of injection molding and injection moldingmachine set-up methods. These and other modifications may be madewithout parting from the ambit of the current invention, the naturewhich may be ascertained from the foregoing description and thedrawings.

1. A method for the automated setting-up of an injection molding machinehaving an injection screw, said machine for manufacturing injectionmolded parts, comprising the steps of: (1) estimating an initialinjection stroke of said injection screw; (2) estimating an initialinjection velocity of said injection screw; (3) generating asubstantially uniform velocity profile from said initial injectionstroke and said initial injection velocity; (4) setting initial packingpressure to a minimal value achievable by said machine; (5)manufacturing a part with said machine, inspecting said part forflashing and short shots, and reducing injection stroke in response toany flashing or increasing injection stroke in response to any shortshots; and (6) manufacturing a part with said machine, inspecting saidpart for flashing and short shots, and reducing injection velocity inresponse to any flashing or increasing injection velocity in response toany short shots; wherein either step (6) is employed after step (5) whenstep (5) is found to have substantially no effect or substantially nofurther effect, and step (5) is employed after step (6) when step (6) isfound to have substantially no effect or substantially no furthereffect, thereby reducing said flashing and short shots, whereby steps(5) and (6) are each employed a plurality of times.
 2. A method asclaimed in claim 1, further including the steps of: (7) determining anoptimum injection velocity profile, including: (i) manufacturing one ofmore parts with said machine; (ii) determining an injection pressureprofile by measuring injection pressure as a function of elapsedinjection time with said machine configured with a substantiallyconstant, desired injection velocity; (iii) measuring injection velocityas a function of elapsed injection time and determining a profile ofsaid measured injection velocity; (iv) defining a mean pressure profilefrom said pressure profile in a regime of substantially constantmeasured injection velocity profile; (v) adjusting said velocity profileover at least a portion of an injection velocity phase in response tosaid pressure profile to reduce differences between said pressureprofile and said mean pressure profile, thereby (ending to lessenirregularities in said pressure profile; (8) modifying a post-velocitycontrol phase intermediate set-up obtained after steps (1) and (7) inresponse to quality defects detected in said parts manufactured withsaid intermediate set-up to reduce said defects; (9) reducing kickbackto an acceptable level to determine a critical packing/holding pressure,including: (i) setting an initial packing/holding pressure to a defaultlow pressure; (ii) performing at lease a partial injection cycle; (iii)determining kickback from changes in screw displacement during said atleast partial injection cycle; (iv) incrementing said initialpacking/holding pressure; and (v) repeating steps (iii) and (iv) ifkickback is unacceptably high until kickback is reduced to apredetermined acceptable level, or initial packing/holding pressurereaches maximum machine pressure; (10) deducing material solidificationrime from measurements of screw displacement to determine an optimalpacking/holding pressure profile, including: (i) defining a holding dimeequal to a predetermined default value; (ii) performing at least apartial injection cycle; (iii) measuring a pressure stroke being thechange in displacement of said screw between packing time and saidholding time; (iv) incrementing said holding time; (v) repeating steps(iii) and (iv) until said pressure stroke stabilizes or a part soproduced is acceptable; (vi) defining a linear relationship betweenscrew displacement and time consistent with screw displacement at saidpacking time and at said holding time, between said packing time andsaid holding time; (vii) defining a gate freeze time as a time ofmaximum difference between said screw displacement and said linearrelationship, thereby providing a value for said gate freeze time frommeasurements of said screw displacement; (11) modifying a post-pressurecontrol phase preliminary set-up obtained after (1) to (10) in responseto defects detected in said parts manufactured with said preliminaryset-up to reduce said defects.
 3. A method as claimed in claim 2,wherein step (iii) of step (9) includes determining kickback frommeasurements of said screw displacement at packing time, including thesteps of: (a) manufacturing one or more parts with said machine; (b)defining as a first pressure the end of velocity control phase pressureand as a second pressure the holding time pressure; (c) defining alinear relationship between packing/holding pressure and time consistentwith said first pressure and said second pressure, between said firstpressure and said second pressure; (d) defining said packing time as atime of maximum difference between measured melt pressure and saidlinear relationship, or as the switchover point if measured meltpressure increases after the switchover point; (e) determining a firstscrew displacement being the minimum displacement of said screw beforesaid packing time within a packing/holding phase and a second screwdisplacement being the displacement of said screw at said packing time;and (f) calculating said kickback from the difference between said firstand second screw displacements, thereby allowing a determination of saidkickback from measurements of said screw displacement at packing time.4. A method as claimed in claim 2, wherein said step (10) includes theadditional steps of: (viii) repeating steps (vi) and (vii), and definingan initial solidification time between said packing time and said gatefreeze time; (ix) repeating steps (vi) and (vii), and defining anintermediate solidification time between said packing time and saidinitial solidification time; and (x) determining an intermediatepressure from the ratio of the screw displacements at said intermediatetime and at said gate freeze time, referenced to said packing time.
 5. Amethod as claimed in claim 1, including: measuring a velocity controlresponse time for said injection molding machine, and employing timesteps equal to or greater than said velocity control response time.
 6. Amethod as claimed in claim 5, wherein said time steps are greater than1.5 times said response time.
 7. A method as claimed in claim 3, whereinstep (10) includes the additional steps of: (viii) repeating steps (vi)and (vii), and defining an initial solidification time between saidpacking time and said gate freeze time; (ix) repeating steps (vi) and(vii), and defining an intermediate solidification time between saidpacking time and said initial solidification time; and (x) determiningan intermediate pressure from the ratio of the screw displacements atsaid intermediate time and at said gate freeze time, referenced to saidpacking time.
 8. A method as claimed in claim 2, wherein nozzle meltpressure, injection cylinder hydraulic pressure, or forward propellingforce applied to said screw is used as a measure of, in place of, or todetermine, injection pressure.
 9. A method as claimed in claim 5,wherein said time steps are equal to 2 times said response time.